Geologic Framework of the Northern Edwards Aquifer, Central Texas

By

COLLINS, EDWARD W.

Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX,

WOODRUFF, C. M., JR.

Woodruff Geologic Consulting, Inc., Austin, TX,

TREMBLAY, THOMAS A.

Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX

The northern segment of the Edwards aquifer comprises Lower Cretaceous Comanche Peak, Edwards, and Georgetown limestones of central Travis, Williamson, and southern Bell Counties, Texas. Geologic and structural-contour maps and cross sections illustrate geologic elements, including the aquifer outcrop belt/recharge area, faults, aquifer stratigraphy, and aquifer-thickness variations that are needed for applications such as (a) identifying aquifer recharge boundaries, (b) recognizing attributes and variations within aquifer strata, (c) making water-management decisions related to groundwater flow and aquifer response for pumpage and recharge, and (d) providing information necessary for land-use activities such as planning and permitting construction projects. Across the study area, normal faults of the Balcones Fault Zone displace Cretaceous limestone, marl, and shale that represent >2,000 ft of shelf and shelf-margin deposition. Faults, having throws between ~800 and <1 ft, strike northeastward to north-northeastward. Composite structural offset is down to the southeast. Fault intensity and composite structural relief decrease northward from Austin, where the composite structural relief is ~1,600 ft. At the northern boundary of the study area in southern Bell County, the composite structural relief is ~600 ft. Faults control the structural position of the porous limestone units that compose the Edwards aquifer, and they bound much of the aquifer recharge zone/outcrop belt. Faults can serve as conduits for groundwater flow, although at some locations faults may displace porous beds against relatively less porous beds, thus causing abrupt changes in groundwater flow paths. The aquifer thickness in central Travis County, ~420 ft, decreases northward to ~260 ft in southern Bell County. Part of this thickness decrease is due to interfingering of Edwards limestone with less porous Walnut marl and argillaceous limestone that are not considered aquifer strata in northern Travis, Williamson, and southern Bell Counties.